The term “lipids” summarizes three classes of natural materials which can be isolated from biological membranes: phospholipids, sphingolipids, and cholesterol, including its derivatives.
These substances are of technical interest in the production of liposomes. Inter alia, such liposomes can be used as containers for active substances in pharmaceutical preparations. In such uses, efficient and stable packaging of the cargo and controllable release of the contents are desirable. Both of these requirements are not easy to combine: the more stable and compact the packaging, the more difficult the release of the entrapped active substance therefrom. For this reason, liposomes changing their properties in response to an external stimulus have been developed. Thermosensitive and pH-sensitive liposomes are well-known. The pH-sensitive liposomes are of special interest, because this parameter undergoes changes even under physiological conditions, e.g. during endocytotic reception of a liposome in a cell, or during passage of the gastrointestinal tract. According to the prior art, pH-sensitive liposomes particularly comprise cholesterol hemisuccinate (CHEMS).
Cholesterol hemisuccinate, in mixture with phosphatidyl ethanolamine, is used to produce pH-sensitive liposomes (Tachibana et al. (1998); BBRC 251, 538-544, U.S. Pat. No. 4,891,208). Such liposomes can enter cells by endocytosis and are capable of transporting cargo molecules into the interior of cells on this route, without doing damage to the integrity of the cellular membrane.
One substantial drawback of CHEMS is its anionic character. Liposomes produced using same have a negative overall charge, being taken up by cells with low efficiency. Despite the transfer mechanism described above, they are barely suitable for the transport of macromolecules into cells.
In WO 00/59474, the prior art describes compounds having a membrane anchor and a cationic and anionic head group, the anionic group being linked to the basic structure via a disulfide bridge. The disulfide bridge can be reduced under physiological conditions, e.g. by contact with the cytosol, the anionic head group then is liberated, and the overall molecule assumes a positive charge, thereby enabling fusion with the cell membrane. The toxicity profile and storage stability of the compounds disclosed in WO 00/59474 are disadvantageous, because cleavage of the disulfide bridges results in free cationic lipids. Disadvantageously, these compounds are known to have a cytotoxic effect.
For the transport of active substances into cells (transfection), the art uses cationic liposomes having a preferably high and constant surface charge. The positive overall charge of such particles leads to electrostatic adherence to cells and subsequently to efficient transport into same. The use of these compounds and of liposomes produced using same remains restricted to in vitro or ex vivo applications, because such positively charged liposomes result in uncontrolled formation of aggregates with serum components.